With more than 30 million HIV-infected individuals, there can be few other more pressing biomedical priorities than to produce an effective vaccine for HIV. Given the important role that cytotoxic T lymphocytes (CTLs) and helper T lymphocytes (HTLs) play in controlling viral replication, it is critical that this vaccine stimulates these cellular responses. Current methods of detecting vaccine-induced immune responses include Intra-Cellular Cytokine Staining (ICS), Enzyme-Linked Spot-Forming Assays (ELISPOT), and tetramer staining. ICS and ELISPOT can be carried out using peptides of 10-15 amino acids in length. However, depending on where the true epitope lies in the synthetic peptide, these peptide sets may not accurately detect the magnitude of the immune response. The identification of minimal optimal epitopes is the best method to accurately assess cellular immune responses. Furthermore, the synthesis of tetramers is absolutely dependent on knowledge of the minimal optimal epitope. Finally, we recently discovered that there are several MHC class l-restricted epitopes in SIV cryptic Open Reading Frames (cORFs). We, therefore, propose to continue our definition of minimal optimal epitopes for common Indian rhesus macaque class I and II molecules in both classical and cryptic ORFs in SIV. PUBLIC HEALTH RELEVANCE (provided by applicant): SIV infection in Indian rhesus macaques is the best animal model for studying HIV-infected humans. SIV challenge of vaccinated Indian rhesus macaques is one of the best defined models available for pre-clinical development of HIV vaccines. Identification of MHC alleles and definition of SIV-specific epitopes is critical in the definition of the immune response in this biomedically important system.